Sept. 12, 2011 -- Testosterone levels drop when men become fathers, and they are lowest for the most devoted dads, new research shows.

While earlier studies had also shown that fathers have lower testosterone levels than men of the same age without children, it has not been clear if men with low testosterone are more likely to become fathers or if fatherhood actually suppresses levels of the male sex hormone.

The new research suggests that the latter is the case.

Researchers say the findings show that unlike most other mammals, human males are biologically hardwired for parenthood.

"The classic idea is that men were the hunters and providers and the females evolved to raise the children," Northwestern University anthropologist and study co-author Christopher W. Kuzawa, PhD, tells WebMD. "I think our study shows pretty clearly that men are also wired for their role as fathers."

Fatherhood Lowers Testosterone

The researchers examined data on close to 500 young Filipino men followed for almost five years. The men were enrolled in a health and nutrition study. Their testosterone levels were measured at the beginning and end of the study.

All of the men were single and had no children when they enrolled in the study. But about a third entered into stable relationships and became first-time fathers during the follow-up.

While the single men with higher testosterone levels at the beginning of the study were more likely to find partners and become fathers, new fathers experienced a drop in levels of the sex hormone greater than drops seen in men of the same age without children over the study period.

Among the fathers in the study, those who were the most involved in the care of their children had lower testosterone than those who were not so involved.

So while testosterone may help men find mates and produce offspring, high levels of the male sex hormone may not be needed and may even be detrimental to caring for a child, the researchers say.

"Fatherhood and the demands of having a newborn baby require many emotional, psychological, and physical adjustments, and our study indicates that a man's biology can change to help meet those demands," anthropologist and lead researcher Lee T. Gettler, tells WebMD.

Testosterone Drop Benefits Baby and Dad

University of Nevada anthropologist Peter B. Gray, PhD, who has also studied the impact of fatherhood on testosterone levels, tells WebMD that the human paternal caregiving role appears to be a relatively recent evolutionary trait.

"If you look at our closest living relatives -- chimpanzees and bonobos [also known as pygmy chimpanzees] -- you see that the males have no role in parenting," he says. "A male may not even know which offspring is his."

He adds that drops in testosterone during times when high levels are not needed for finding a mate and reproduction may benefit both males and their offspring.

High testosterone has been linked to aggression, as well as to a possible increased risk for prostate and testicular cancer, stroke, heart disease, and even memory decline, Gray says.

"A decline in testosterone associated with the parental role is really not a bad thing," he says. "It is part of an adaptive attunement in our physiology that may be beneficial for a man's overall health and providing good parental care."